1. Field of the Invention
The present invention relates to a radio-frequency power amplifier using a transistor, and particularly to a radio-frequency power amplifier including a protection circuit.
2. Description of the Background Art
A radio-frequency power amplifier of a mobile communication terminal such as a mobile phone includes a multistage amplifier using, for two to three stages, transistors made of compound semiconductors. Further, in recent years, as a device for an amplifier, a hetero bipolar transistor (hereinafter referred to as an “HBT”) capable of single power supply operation is mainly used. The HBT has the advantage of having higher power density per unit area than that of a field effect transistor and thus being reducible in size, but also has the problem of having lower ruggedness at the time of load mismatch than that of the field effect transistor.
The ruggedness at the time of load mismatch will be described below. Mobile communication terminals of a GSM (Global System for Mobile Communications), which is the world's currently most common mobile phone communication system, and some mobile communication terminals of a UMTS (Universal Mobile Telecommunications System), which has recently started to become common, do not use an isolator for stabilizing the load impedance of a radio-frequency power amplifier, due to reducing the mobile communication terminals in size and the like. Consequently, when a termination condition of an antenna changes, a load condition of the radio-frequency power amplifier also changes.
FIG. 21 is a general diagram showing a dynamic load line and a safe operating area at the time of load mismatch, of the HBT. In FIG. 21, Ic represents a collector current and Vc represents a collector voltage. In FIG. 21, when the load condition changes, the dynamic load line changes significantly. Then, the HBT is destroyed when the dynamic load line of the HBT goes beyond the safe operating area, i.e., reaches a thermal destruction area or a breakdown-voltage destruction area. More specifically, the HBT is thermally destroyed when the dynamic load line exceeds a thermally safe operating limit, and the HBT is breakdown-voltage-destroyed when the dynamic load line exceeds a breakdown voltage limit.
FIG. 22 is a diagram showing the dynamic load line at the time of load mismatch, of a final-stage HBT which is included in a multistage amplifier and to which the collector voltage exceeding a rated value is applied (hereinafter referred to as an “overvoltage condition”) due to the change of a power supply voltage and the like. As shown in FIG. 22, in an overvoltage condition, the dynamic load line shifts in the direction of increasing the voltage (Vc) and breakdown-voltage destruction is caused. To prevent such destruction at the time of load mismatch in the overvoltage condition from occurring, a method is proposed for limiting the input power to a final-stage HBT, by incorporating a protection circuit which senses the collector voltage of the final-stage HBT and reduces the base voltage of a first-stage HBT in the overvoltage condition (see Japanese Laid-Open Patent Publication No. 2005-64658).
FIG. 23 is a diagram showing the dynamic load line of the first-stage HBT which is included in the multistage amplifier and of which the base voltage is reduced by the above-described protection circuit. As shown in FIG. 23, the base voltage is reduced, whereby the dynamic load line of the first-stage HBT shifts in the direction of reducing the collector current (Ic). Consequently, the output power of the first-stage HBT is reduced, and thus the power inputted to the final-stage HBT is reduced. FIG. 24 is a diagram showing the dynamic load line at the time of load mismatch, of the final-stage HBT, in the case where, in the overvoltage condition, the base voltage of the first-stage HBT is reduced by the protection circuit and thus the input power to the final-stage HBT is reduced. As shown in FIG. 24, the input power to the final-stage HBT is reduced, whereby the dynamic load line of the final-stage HBT becomes smaller. Consequently, the dynamic load line of the final-stage HBT does not go beyond the safe operating area, thus it is possible to prevent the final-stage HBT from being destroyed.
However, the method disclosed in Japanese Laid-Open Patent Publication No. 2005-64658 has the following problem. FIG. 25 is a diagram showing the dynamic load line of the first-stage HBT of which the base voltage is reduced by the protection circuit and to the base of which a radio-frequency signal exceeding the rated power is then inputted. As shown in FIG. 25, when the radio-frequency signal exceeding the rated power is inputted to the base of the first-stage HBT (hereinafter referred to as “at the time of excessive input”), the dynamic load line of the first-stage HBT shifts in the direction of increasing the collector current and also becomes larger due to the increase of the input power thereto. That is, the output power of the first-stage HBT is increased. This results from a phenomenon that the base voltage of the first-stage HBT is increased due to a voltage swing of the radio-frequency signal inputted thereto. FIG. 26 is a diagram showing the dynamic load line at the time of load mismatch, of the final-stage HBT, in the case where the dynamic load line of the first-stage HBT shifts in the direction of increasing the collector current in FIG. 25. Since the output power of the first-stage HBT cannot be reduced when the first-stage HBT is at the time of excessive input (see FIG. 25), the input power to the final-stage HBT is not reduced. Therefore, as shown in FIG. 26, the dynamic load line at the time of load mismatch, of the final-stage HBT becomes larger, and as a result, the final-stage HBT is destroyed when the dynamic load line goes beyond the safe operating area. As described above, in the method disclosed in Japanese Laid-Open Patent Publication No. 2005-64658, when the base of the first-stage HBT is at the time of excessive input, the output power of the first-stage HBT cannot be reduced even if the base voltage of the first-stage HBT is reduced by the protection circuit, and thus the final-stage HBT may be destroyed.
FIG. 27 is a diagram showing, in a two-stage power amplifier employing the method disclosed in Japanese Laid-Open Patent Publication No. 2005-64658, the output power of a final-stage HBT, in the case where the radio-frequency signal of the rated power is inputted to the base of a first-stage HBT and in the case where the radio-frequency signal exceeding the rated power is inputted thereto. Note that: the operating frequency of the two-stage power amplifier is 0.9 GHz; regarding the device size of the first-stage HBT, the emitter area is 200 um2; regarding the device size of the final-stage HBT, the emitter area is 800 um2; the collector voltage (Vc) of the first-stage HBT is 1.8 V (fixed); the rated collector voltage of the final-stage HBT is 3.5 V; and the input/output impedance of the two-stage power amplifier is matched to 50 Ω by an input/output /interstage matching circuit. As shown in FIG. 27, when the input power (Pin) to the first-stage HBT is 0 dBm, the output power of the first-stage HBT is reduced in the overvoltage condition in which the collector voltage (Vc) of the final-stage HBT exceeds the rated collector voltage of 3.5 V. As a result, the output power (Pout) of the final-stage HBT is reduced. However, it is indicated that when the input power (Pin) to the first-stage HBT is +5 dBm (in an excessive input condition), the output power of the first-stage HBT is not reduced. As a result, in the overvoltage condition in which the collector voltage (Vc) of the final-stage HBT exceeds the rated collector voltage of 3.5 V, the output power (Pout) of the final-stage HBT is increased.